A semiconductor memory includes a volatile memory such as a DRAM (Dynamic Random Access Memory) and a non-volatile memory such as a flash memory. As the non-volatile memory, a NAND flash memory or the like is known, and as a device capable of attaining further miniaturization, a ReRAM (Resistance RAM) is attracting attention.
A ReRAM uses a variable resistor, the resistance value of which is changed by applying a pulse voltage thereto, as a resistance element. Typically, the variable resistor has the structure in which two or more metal oxide layers having different oxidation degrees, that is, different resistivities are sandwiched by upper and lower electrodes. As a method of forming a layer structure of oxides having different oxidation degrees, there is known a method of forming a metal oxide by so-called reactive sputtering in which a target made of metal is sputtered in an oxygen atmosphere. For example, Patent Document 1 discloses a method of layering an n-type transition metal oxide layer having a high oxygen concentration and a p-type transition metal oxide layer having a low oxygen concentration by reducing a supply amount of oxygen while sputtering a target made of metal.
Patent Document 1: Japanese Patent Application Laid-open No. 2008-244018 (paragraphs [0036] and [0037])